HydroGuard uses four electrodes. The outer pair sends a small alternating current through the upper arm. The inner pair measures the resulting voltage. Separating these two jobs makes the measurement less sensitive to the resistance where each electrode touches the skin.
The board is 100 × 100 mm, far larger than anything anyone would wear. That is the point. At this size every net can reach a test point, the controller sits in a socket rather than soldered down, and an oscilloscope probe fits between the parts. A measurement none of us had made before is hard enough without doing it under a microscope.
The first board gives the MAX30001 its own 1.8 V supply and clock. Every electrode path has a 1 kΩ series resistor, and the reference voltage has a test point. The ESP32-C6 module is removable, which helped when we needed to probe signals or change the controller.
Four electrodes on an elastic band
We use 12 mm sew-on snaps to connect the electrodes. They are easy to replace and easy to position on an elastic band. The band holds them flat against the arm without needing a custom adhesive patch.
What it cost to have made
We ordered the boards from JLCPCB on 5 July and had them in hand on 24 July. Four layers, 100 × 100 mm, 1.6 mm FR-4, 1 oz outer copper and 0.5 oz on the inner layers, smallest via 0.3 mm, inspected to IPC Class 2.
The finish is ENIG rather than the cheaper HASL. The MAX30001 comes in a wafer-level package with 0.5 mm pitch, and ENIG gives a flat surface to place that on. HASL leaves domed pads, which is fine for a connector and not fine for this.
We had five bare boards made and two of them assembled, top side only. The MAX30001 was pre-ordered ahead of the run rather than bought with it, because it is the one part on the board we did not want to leave to stock availability on the day. The controller module and the I2C headers were deliberately left off the assembly and soldered by hand afterwards, which is also what made the module swappable.
Both assembled boards came back exactly as drawn and both worked. Nothing was misplaced, no joint needed reworking, and the fine-pitch part under the microscope looked like the footprint we sent. For a board whose whole purpose was to let us trust the measurement, that mattered more than the price did.
A bare board was €4.30 and an assembled one €35.04. With shipping and 19 percent import VAT the whole order came to €126.37. That is the real number for getting a four-layer board with a fine-pitch part on it built, at a quantity that makes no sense to a factory.